Multi conductor power cables, also known as portable cables, are known. Although this is not an exhaustive listing of their applications, they are used extensively in mining operations for power excavating, drilling and transporting equipment both in the mine and on the surface. They are typically coiled on and off a reel as the machine moves. Such reels are generally described in U.S. Pat. No. 4,664,331. Other uses for such portable cables are for cranes and other heavy equipment on docks and construction sites and also as temporary power cables used by utility companies and others. During use, these cables are dragged across the ground, roadways, tracks and other obstacles and subject to damage.
If a multi conductor power cable is damaged, this may result in electrical faults that could cause fire, arcing and explosion of combustible gasses and injury to persons by burning or electrical shock.
Commonly assigned U.S. Pat. No. 6,402,993 discloses a cable construction suitable for higher voltages of about 15 Kv. This patent is illustrative of a basic construction wherein a conductor is shielded with a semi-conductive polymer compositions or possibly conductive tape, then also wrapped with copper wire, copper braid or metal tape. The conductive polymer composition in this case equalizes the electrical stresses that would occur under each wire and prevents an electrical discharge at the overlaps of the tape. At higher voltages the fault current jumps directly to the copper tapes or wires. Cables may also have semi conductive water blocking layers as in U.S. Pat. No. 6,455,769.
Shielded lower voltage cables are also known where braided copper or aluminum wires or copper tapes are placed over the power and ground conductors. These constructions have several disadvantages in that they are much more expensive to produce, are less flexible and tend to have a shorter life if flexed as the braid breaks or cuts into the insulation of the conductor. They have a larger cross section and are heavier so less length can be put on a reel and are also harder to terminate.
Lower voltage cables may or may not have a designated ground conductor or an uninsulated ground conductor.
Ground check cables are known. A ground-check conductor is an insulated conductor which is used to “check” the grounding conductor. Approximately one ampere of current is sent down the ground check. The GC is mechanically tied in to the frame of the machine as is the grounding conductor. A simple series circuit is formed and if either the ground or the ground check is broken, the circuit is incomplete and the circuit breaker in the power center will trip.
There are constructions wherein shields on shielded cables carry the fault current to the ground conductor. Typically a relay or monitoring circuit interrupts the voltage to the cable which is then repaired or replaced. U.S. Pat. No. 6,801,117 discloses a cable with thermal and damage sensor wires. Damage is sensed when the sensor wire is broken. A disadvantage of this construction is that a sensor wire could break while the conductor is not exposed. Also, a complicated circuit is required to monitor the sensors and a signal must be sent from the other end of the wire to the monitoring circuit for it to detect an interruption. U.S. Pat. No. 4,785,163 discloses an un-grounded heating cable where two polymer composition layers are separated by an insulation. If contact is made between the two layers damage is sensed. U.S. Pat. No. 6,784,371 discloses a cable with sensing wires to detect a substance inside the cable. Other cables are known with temperature and moisture sensors. In all these cables a signal is generated that must be processed and a determination made to shut off the cable.
Other solutions to the problem of cable damage have been proposed such as so called “self healing” cables wherein a polymer composition seals and repairs damage in a cable. See, for example, U.S. Pat. Nos. 6,184,473 and 5,313,020.
Ground fault current interrupters are known that can detect small currents leaking to ground. In many applications ground fault current interrupters are set at a high level to avoid false trips due to moisture in the insulation conducting small currents or one or more areas of small damage that are difficult to find and fix but would otherwise trip the circuit breaker.